Zavitsas’ model of aqueous NaF solution activities utilizing hydration numbers reported from Dielectric Relaxation spectroscopy

Abstract

Dielectric Relaxation Spectroscopy (DRS) measures the number of water molecules irrotationally bound to electrolytes, which is a similar concept to the hydration number in Zavitsas’ model of electrolyte solution thermodynamics. Zavitsas’ model assumes that some water molecules are bound to electrolytes so strongly that they act as part of the electrolyte rather than the solvent. Most monovalent electrolytes have linear water activity versus concentration plots at low concentrations and many sparingly soluble electrolytes like NaF are only soluble in the linear region. Multiparameter models fit to this data are statistically overparametrized. The present study relieves overparameterization by substituted the number of irrotationally bound water molecules determined by DRS for the hydration number in Zavitsas’ model and then optimized the NaF extent of dissociation (ie) to best fit water activity data. This ie value was consistent with and could be predicted by The Law of Matching Water Affinities. Using the resulting model parameters, the solubility of NaF was predicted in aqueous NaNO3 solutions without having to optimize any parameters to the solubility data. These results demonstrates that DRS can be used to parametrize Zavitsas’ model.